Summary of parent grant Aims: R01 NS091836, Mechanisms Underlying Excitability Regulation of Motoneuron Types in ALS (Amyotrophic Lateral Sclerosis). Motoneuron (MN) excitability remains tightly linked to ALS pathogenesis. Further, large, fast-twitch MNs (F-MNs) die before small, slow-twitch MNs (S-MNs), suggesting that S-MNs are less vulnerable to ALS. We showed that S-and F-MNs differ in intrinsic expression of SK channels. As SK channels contribute to MN excitability regulation, our parent grant (Aim 1) Determined the role of SK channels in the excitability regulation of S- vs. F-MNs in ALS via computational models; (Aim 2) Empirically verified model predictions on altered SK channels via immunohistochemistry; and (Aim 3) Showed that SK modulators extended survival in ALS mice. New Work Proposed: We now hypothesize SK involvement in a pathology common to ALS and Alzheimer?s Disease (AD): Transactive response DNA-binding protein 43 (TDP-43) pathology is seen in >90% of ALS cases and in >50% of AD patients (75% in high-stage AD). Also, AD pathology is found in 30% of ALS/TDP-43 patients. While AD and TDP43/FTLD have been studied separately, one recent study showed interlinked pathogenesis in 95% of patients with AD and TDP-43, while others show interacting TDP-43 and amyloid beta pathologies. Additionally, early neuronal and network hyperexcitability have been mechanistically linked to both ALS and AD. The new rNLS8 TDP-43 mouse line recapitulates TDP-43 aggregates in the brain and spinal cord, leading to neuronal loss similar to what is seen in both ALS and AD. This allows us to study similar, potentially interactive pathologies of AD and ALS within the same animals. Thus, we propose to assess the general significance of SK channel downregulation as a mechanism contributing to TDP43-implicated ALS and AD pathogenesis in rNLS8 TDP-43 mice. We will examine spinal motoneurons (Aim 1) and subcortical neurons in the CA1 and dentate gyrus areas of the hippocampus, and the amygdala (Aim 2), both at end-stage. These subcortical areas display TDP-43 aggregates in ALS and AD. Specific Aim 1: Determine the involvement of SK channel downregulation in the degeneration of spinal MNs in the rNLS8 TDP-43 mouse model. Measure the expression levels of SK2 and SK3 channel isoforms in spinal MNs of rNLS8 TDP-43 mice vs. WT littermates via immunohistochemistry. Our hypothesis is that the expression level of SK2 and SK3 channel isoforms are reduced at end-stage in MNs of rNLS8 mice relative to WT. Specific Aim 2: Determine the involvement of SK channel downregulation in the degeneration of subcortical neurons involved in ALS and AD pathologies in the rNLS8 TDP-43 mouse model. Measure expression levels of SK2 and SK3 channel isoforms in subcortical neurons of rNLS8 TDP-43 mice vs. WT via immunohistochemistry in the CA1 and dentate gyrus areas of the hippocampus, and in the amygdala. This investigation has potential to identify a broad neurodegenerative mechanism common to different mouse models of ALS (Aim 1) and in AD (Aim 2); and to identify a potential mechanistic association between motor function loss and cognitive impairment.